The electroretinogram (ERG) of the rhodopsin knockout
(rho−/−) mouse of Humphries et al. (1997) (Humphries
et al., 1997) was studied for evidence of light-evoked
rod activity and to describe the cone function. The rho−/−
retina develops normal numbers of rod and cone nuclei,
but the rods have no outer segments, and no rhodopsin is
found by immunohistochemistry. The dark-adapted ERG threshold
was elevated 4.7 log units above wild-type (WT) control
mice, indicating that any residual rod responses were reduced
>50,000-fold, consistent with a complete functional
knockout. The dark-adapted rho−/− ERG had a
cone waveform, and the spectral sensitivity peaked near
510 nm for both dark-adapted and light-adapted conditions,
without evidence of a Purkinje shift. The light-adapted
ERG b-wave amplitude of young rho−/−
mice was the same as WT. The amplitude remained steady
up to postnatal day P47, but thereafter it declined to
only 1–2% by P80 when no cone outer segments remained.
Cone b-wave threshold of dark-adapted rho−/−
mice was −1.07 ± 0.39 log cd-s/m2
(n = 17), which is 1.27 log units more sensitive
than light-adapted thresholds against a rod-suppressing
Ganzfeld background of 1.61 log scotopic cd/m2.
This indicates that dark-adapted WT responses to still
dimmer stimuli are exclusively rod driven with minimal cone
intrusion. Above this cone threshold intensity, the dark-adapted
b-wave of WT will be a summation of rod and cone
responses. Threshold versus intensity (TVI) studies gave no
evidence of a rod influence on the mouse cone b-wave.
A Purkinje shift in the spectral sensitivity of grey squirrels